1. Field of the Disclosure
The present application relates to an organic light emitting display device. More particularly, the present application relates to an organic light emitting display device adapted to enhance an aperture ratio by allowing a storage electrode in a pixel region to become a transparent electrode, and to a fabricating method thereof.
2. Description of the Related Art
The general public has shown a keen interest in information display, and public demand for the use of portable information media is increasing. Flat panel display devices having features of thinness, lighter weight, and suitability to replace existing cathode ray tubes (CRTs) are being actively researched and commercialized.
In the flat panel display field, liquid crystal display (LCD) devices with features of light weight and low power consumption have been attracting public attention. However, the LCD devices have low brightness, a low contrast ratio and a narrow viewing angle because of being used as a light receiving element not a luminous element. As such, new display devices that are suitable to overcome the disadvantages of the LCD devices are actively being developed.
An organic light emitting display (OLED) device corresponding to one of the new display devices is a self-luminous device. In other words, the OLED device does not require any backlight. As such, the OLED device can provide advantages of a wider viewing angle, a superior contrast ratio, lower power consumption and so on, compared to the LCD device. Also, the OLED device can not only become thinner but also reduce weight. Moreover, the OLED device can be driven by a low DC (direct current) voltage, enhance the response time, and reduce the fabrication cost.
Such an OLED device can be fabricated using only deposition and encapsulation processes, unlike the LCD device and a plasma display panel (PDP). As such, the fabrication procedure of the OLED device is very simple. If pixels are driven in an active matrix mode which uses thin film transistors within each pixel as switching elements, the OLED device can obtain the same brightness as the LCD device even though it uses a small current. Therefore, the OLED device can reduce power consumption and be easy to realize high definition and a large-sized screen.
The basic structure and operation properties of an organic light emitting diode formed in each pixel region of the organic light emitting display device will now be described with reference to attached drawings.
FIG. 1 is a diagram illustrating an emission principle of an organic light emitting diode forming an ordinary organic light emitting display device.
The ordinary organic light emitting display device includes an organic light emitting diode shown in FIG. 1. The organic light emitting diode includes an organic compound layer 30a through 30e interposed between an anode electrode 18 corresponding to a pixel electrode and a cathode electrode 28 corresponding to a common electrode.
The organic compound layer 30a through 30e includes a hole injection layer 30a, a hole transport layer 30b, an emission layer 30c, an electron transport layer 30d and an electron injection layer 30e. 
If a driving voltage is applied between the anode electrode 18 and the cathode electrode 28, holes passing through the hole transport layer 30b and electrons passing through the electron transport layer 30d are drifted into the emission layer 30c, thereby generating excitons. In accordance therewith, the emission layer 30c can emit visible light.
The organic light emitting display device includes pixels which each have the organic light emitting diode of the above-mentioned structure and are arranged in a matrix shape. Such an OLED device selectively controls the pixels using data voltages and scan voltages, in order to display an image.
FIG. 2 is an equivalent circuit diagram showing a pixel region of an ordinary organic light emitting display device.
Referring to the equivalent circuit of FIG. 2, an organic light emitting display device of an active matrix type includes a pixel of a 2T1C mode as an example. The pixel of the 2T1C mode includes two thin film transistors and a single capacitor.
Such a pixel of the active matrix type organic light emitting display device includes an organic light emitting diode OLED, a switching thin film transistor SW, a driving thin film transistor DR and a storage capacitor Cst within a pixel region which are defined by a data line DL and a gate line GL crossing each other.
The switching thin film transistor SW is turned-on in response to a scan pulse from the gate line GL and forms a current path between its source and drain electrodes. During the turned-on period of the switching thin film transistor SW, a data voltage is applied from the data line DL to the driving thin film transistor DR and the storage capacitor Cst via the source electrode and the drain electrode of the switching thin film transistor SW.
The driving thin film transistor DR controls the quantity of a current flowing through the organic light emitting diode OLED according to the data voltage applied to its gate electrode. The storage capacitor Cst stores a different voltage between the data voltage and a low power supply voltage VSS, and constantly maintains the different voltage for a single frame interval.
However, the storage capacitor Cst of the organic light emitting display device of the related art is prepared by forming a first storage electrode in a storage capacitor region at the formation of gate electrodes of the switching and driving thin film transistors SW and DR, and then forming a second storage electrode overlapping with the first storage electrode at the formation of the source/drain electrodes of the switching and driving thin film transistors. As such, all the storage electrodes must be formed from an opaque metal.
In other words, the storage electrodes positioned within the pixel region must be formed from an opaque metal. Due to this, the aperture ratio of the pixel must deteriorate. Also, it is difficult to increase the capacitance of the storage capacitor through the expansion of the storage electrodes.